Rescue workers at the site of the Neva express train explosion (ITAR-TASS)
August 15, 2007 (RFE/RL) -- Two days after a bomb attack on a Russian express train left 60 people injured, it remains unclear who is responsible for the blast.

The Russian Prosecutor-General's Office says the August 13 explosion on the Neva express train running from St. Petersburg to Moscow was caused by a homemade bomb packed with the equivalent of 2 kilograms of TNT.

The explosion occurred near the city of Novgorod, about 500 kilometers northwest of Moscow. The blast, which investigators believe was detonated by remote control, tore a gap in the rail line some 30 meters before a bridge.

The train, which was traveling at approximately 200 kilometers per hour, had enough momentum to cross the bridge before derailing.

Extremists 'Most Likely' Suspects

Russian officials say they are looking for suspects in the blast. Reports say experts had drawn sketches of at least one suspect based on eyewitness descriptions of possible participants seen prior to the blast.

Russia's Interfax news agency quoted unnamed investigators as saying nationalist extremists are the most likely culprits in the train bombing.

"Investigators and detectives are working in several directions. However, the involvement of extremist nationalist organizations in the terrorist act is one of the priorities," the source is quoted as saying.

The Russian Prosecutor-General's Office also raised the possibility of Chechen involvement.

Spokeswoman Marina Gridnyova told Interfax that prosecutors were investigating all avenues, and did not rule out that "Caucasians" were behind the attack.

Chechen Rebels Claim Bombing

A man identifying himself as a member of a Chechen extremist organization late on August 14 claimed the group was responsible for the attack.

The man, who identified himself as the deputy commander of the Riyadus Salikhin organization, told RFE/RL's North Caucasus Service in a phone call that the bombing, which injured more than 60 people, was in retaliation for Russia's role in Chechnya.

The identity of the caller and his claims could not be verified.

Riyadus Salikhin (Gardens of the Righteous) was formerly led by rebel commander Shamil Basayev, who was killed by Russian special forces in July 2006. The group has been linked to a number of terrorist acts.

No composite sketches of suspects have been released to the public. But a handful of Russian and Chechen news sites have quoted unnamed sources as saying one sketch shows a man who is Slavic in appearance.

"The Moscow Times" newspaper reported that electric cables found at the site of the bombing indicate ultranationalists might be behind the attack.

Similar cables were found at the site of two previous bombings, both tied to ultranationalists.

Agencies are reporting that authorities are questioning a regional leader from the ultranationalist Movement Against Illegal Immigration in connection with the bombing.

Aleksandr Belov, the head of the group, told Interfax that investigators questioned Sergei Larionov, the regional branch head in Novgorod Oblast.